Study of the role of abyssal ocean stratification in the rearrangement of potential vorticity through the water column

Abstract. Observations of abyssal variability performed in the Ionian Sea (Mediterranean Sea) have revealed the presence of a dense, stable abyssal layer, whose thermohaline and dynamical properties changed drastically over a decade. Building upon these available observations, we aim to investigate the role that stratification can play in the transmission of vorticity throughout the water column to the abyss and, in turn, in the redistribution of energy stored in the deep sea, with a set of stationary states. A quasi-geostrophic level model equipped with four coupled layers, a free surface, and a mathematical artifice for parameterizing decadal time evolution has been considered, proving that the relative-thickness and relative-density differences among the layers are the two critical factors that determine the dynamical characteristics of this rearrangement. The variability in ocean stratification is a relevant aspect that can activate deep and intermediate dynamics, engaging in the propagation and stabilization of signals throughout the water column. This demonstrates the non-negligible active connection between the dynamics of the bottom layers and the surface. The theoretical framework and parameterization used are based on specific observations made in the Ionian Sea over the last decades but retain general applicability to all ocean basins that are characterized by the presence of a stratified, dense water mass in their deep and intermediate layers.